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Slo-mo tremors make big Tokyo quakes more likely

By Jeff Hecht

THE people of Tokyo have long lived in fear of another great earthquake – fears that seem to be increasingly justified.

Slow-motion earthquakes have become more common near the city in the last few years, causing tectonic stresses to build up. And the after-effects of the Tohoku megaquake, which struck off the north-east coast of Japan in 2011, causing a devastating tsunami, are also pushing the area towards a big quake. But seismologists can’t predict when it might occur, nor which part of the region’s complex fault system will break.

Shinzaburo Ozawa of the Geospatial Information Authority of Japan in Tsukuba used GPS sensors to track the surface motion of the Boso peninsula, the eastern side of Tokyo Bay. Between 28 December 2013 and 10 January 2014, he detected centimetre-scale shifts. These were caused by two tectonic plates slipping by about 10 centimetres in total. The motion released as much energy as a magnitude 6.5 earthquake, but caused no damage because it was spread over two weeks.

Seismographs don’t record such slow slips, so they went unnoticed until GPS came along, says Heidi Houston of the University of Washington in Seattle, who wasn’t involved in the research.

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The concern is that the slow-slip quakes seem to be occurring more frequently, a sign of increasing tectonic stress in the region. The latest slip came only 2.2 years after the previous one, a month-long slip in late 2011. The first slips detected, beginning in 1996, were 6.4 years apart (Geophysical Research Letters, doi.org/sfq)

The Boso slip is a reminder that “it is essential to keep a close eye on the deformation and seismicity in this region”, says Roland Burgmann of the University of California, Berkeley.

But Goran Ekstrom at Columbia University in New York says that because so few slow-slip events have been recorded, the increase in frequency could just be natural variation.

Other evidence also suggests that a big Tokyo quake is on the way. After the Tohoku megaquake, the frequency of seismic activity in the Tokyo area jumped tenfold, before levelling off at three times the pre-quake rate.

Based on that increase, a study last year estimated that there was a 17 per cent chance of a large quake under Tokyo between March 2013 and March 2018. That is two-and-a-half-times higher than if the Tohoku quake hadn’t happened (Geophysical Research Letters, doi.org/scz).

The Tohoku quake has “completely rearranged the whole system in north-east Japan”, says Burgmann. The findings “definitely point to the very complicated area around Tokyo becoming a zone of greater hazard”, he says.

Four tectonic plates meet in the Tokyo area, and as a result it has experienced several quakes above magnitude 7 in the past 400 years. The largest was the Genroku quake, estimated at magnitude 8.2, that killed 2300 people on 31 December 1703.

However, the deadliest was the magnitude 7.9 Great Kanto earthquake on 1 September 1923, pictured left, which killed 100,000 people – with some help from a typhoon.

This article appeared in print under the headline “Slo-mo tremors make big Tokyo quake more likely”